Generate less WAL during GiST, GIN and SP-GiST index build.

Instead of WAL-logging every modification during the build separately,
first build the index without any WAL-logging, and make a separate pass
through the index at the end, to write all pages to the WAL. This
significantly reduces the amount of WAL generated, and is usually also
faster, despite the extra I/O needed for the extra scan through the index.
WAL generated this way is also faster to replay.

For GiST, the LSN-NSN interlock makes this a little tricky. All pages must
be marked with a valid (i.e. non-zero) LSN, so that the parent-child
LSN-NSN interlock works correctly. We now use magic value 1 for that during
index build. Change the fake LSN counter to begin from 1000, so that 1 is
safely smaller than any real or fake LSN. 2 would've been enough for our
purposes, but let's reserve a bigger range, in case we need more special
values in the future.

Author: Anastasia Lubennikova, Andrey V. Lepikhov
Reviewed-by: Heikki Linnakangas, Dmitry Dolgov

1 files changed, 12 insertions, 8 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index c6ca96079c1..e3a3110716d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5242,7 +5242,7 @@ BootStrapXLOG(void)
 	ControlFile->time = checkPoint.time;
 	ControlFile->checkPoint = checkPoint.redo;
 	ControlFile->checkPointCopy = checkPoint;
-	ControlFile->unloggedLSN = 1;
+	ControlFile->unloggedLSN = FirstNormalUnloggedLSN;
 
 	/* Set important parameter values for use when replaying WAL */
 	ControlFile->MaxConnections = MaxConnections;
@@ -9781,12 +9781,11 @@ xlog_redo(XLogReaderState *record)
 	}
 	else if (info == XLOG_FPI || info == XLOG_FPI_FOR_HINT)
 	{
-		Buffer		buffer;
-
 		/*
 		 * Full-page image (FPI) records contain nothing else but a backup
-		 * block. The block reference must include a full-page image -
-		 * otherwise there would be no point in this record.
+		 * block (or multiple backup blocks). Every block reference must
+		 * include a full-page image - otherwise there would be no point in
+		 * this record.
 		 *
 		 * No recovery conflicts are generated by these generic records - if a
 		 * resource manager needs to generate conflicts, it has to define a
@@ -9798,9 +9797,14 @@ xlog_redo(XLogReaderState *record)
 		 * XLOG_FPI and XLOG_FPI_FOR_HINT records, they use a different info
 		 * code just to distinguish them for statistics purposes.
 		 */
-		if (XLogReadBufferForRedo(record, 0, &buffer) != BLK_RESTORED)
-			elog(ERROR, "unexpected XLogReadBufferForRedo result when restoring backup block");
-		UnlockReleaseBuffer(buffer);
+		for (uint8 block_id = 0; block_id <= record->max_block_id; block_id++)
+		{
+			Buffer		buffer;
+
+			if (XLogReadBufferForRedo(record, block_id, &buffer) != BLK_RESTORED)
+				elog(ERROR, "unexpected XLogReadBufferForRedo result when restoring backup block");
+			UnlockReleaseBuffer(buffer);
+		}
 	}
 	else if (info == XLOG_BACKUP_END)
 	{